Semiconductor device and manufacturing method thereof

ABSTRACT

In a method for manufacturing a semiconductor device, a fin structure including a first semiconductor layer, an oxide layer disposed over the first semiconductor layer and a second semiconductor layer disposed over the oxide layer is formed. An isolation insulating layer is formed so that the second semiconductor layer of the fin structure protrudes from the isolation insulating layer while the oxide layer and the first semiconductor layer are embedded in the isolation insulating layer. A third semiconductor layer is formed on the exposed second semiconductor layer so as to form a channel.

TECHNICAL FIELD

The disclosure relates to a semiconductor integrated circuit, moreparticularly to a semiconductor device having a fin structure and itsmanufacturing process.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as a finfield effect transistor (Fin FET). Fin FET devices typically includesemiconductor fins with high aspect ratios and in which channel andsource/drain regions of semiconductor transistor devices are formed. Agate is formed over and along the sides of the fin structure (e.g.,wrapping) utilizing the advantage of the increased surface area of thechannel and source/drain regions to produce faster, more reliable andbetter-controlled semiconductor transistor devices. In some devices,strained materials in source/drain (S/D) portions of the FinFETutilizing, for example, silicon germanium (SiGe), silicon phosphide(SiP) or silicon carbide (SiC), may be used to enhance carrier mobility.Further, channel on oxide structures have been proposed to improvecarrier mobility and to maintain a straight fin profile. In addition,strained materials in source/drain (S/D) portions of the Fin FETutilizing selectively grown silicon germanium (SiGe) may be used toenhance carrier mobility. For example, compressive stress applied to achannel of a PMOS device advantageously enhances hole mobility in thechannel. Similarly, tensile stress applied to a channel of an NMOSdevice advantageously enhances electron mobility in the channel.However, there are challenges to implementation of such features andprocesses in complementary metal-oxide-semiconductor (CMOS) fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is an exemplary process flow chart of a semiconductor FET deviceaccording to one embodiment of the present disclosure.

FIGS. 2-8 are exemplary processes for manufacturing a semiconductor FETdevice according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“made of” may mean either “comprising” or “consisting of.”

FIG. 1 is an exemplary flow chart for manufacturing a semiconductor FinFET device according to one embodiment of the present disclosure. Theflow chart illustrates only a relevant part of the entire manufacturingprocess. It is understood that additional operations may be providedbefore, during, and after the operations shown by FIG. 1, and some ofthe operations described below can be replaced or eliminated foradditional embodiments of the method. The order of theoperations/processes may be interchangeable.

In S101 of FIG. 1, stacked layers of semiconductor materials are formedover a substrate as shown in FIG. 2. The stacked layers of semiconductormaterials are formed over a substrate 10 and include a firstsemiconductor layer 15, an intermediate semiconductor layer 20, and asecond semiconductor layer 30.

The substrate 10 is, for example, a p-type silicon substrate with animpurity concentration in a range of about 1×10¹⁵ cm⁻³ and about 3×10¹⁵cm⁻³. In other embodiments, The substrate 10 is an n-type siliconsubstrate with an impurity concentration in a range of about 1×10¹⁵ cm⁻³and about 3×10¹⁵ cm⁻³. The Si substrate 10 has a (100) upper surface insome embodiments.

Alternatively, the substrate 10 may comprise another elementarysemiconductor, such as germanium; a compound semiconductor includingIV-IV compound semiconductors such as SiC and SiGe, III-V compoundsemiconductors such as GaAs, GaP, GaN, InP, InAs, InSb, GaAsP, AlGaN,AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof.In one embodiment, the substrate 10 is a silicon layer of an SOI(silicon-on-insulator) substrate. When an SOI substrate is used, the finstructure may protrude from the silicon layer of the SOI substrate ormay protrude from the insulator layer of the SOI substrate. In thelatter case, the silicon layer of the SOI substrate is used to form thefin structure. Amorphous substrates, such as amorphous Si or amorphousSiC, or insulating material, such as silicon oxide may also be used asthe substrate 10. The substrate 10 may include various regions that havebeen suitably doped with impurities (e.g., p-type or n-typeconductivity).

The first semiconductor layer 15 may be part of the substrate implantedwith impurities. The ion implantation is performed to prevent apunch-through effect. The dopants are, for example boron (BF₂) for ann-type Fin FET and phosphorus for a p-type Fin FET. The firstsemiconductor layer 15 becomes a well layer of a Fin FET.

In some embodiments, the first semiconductor layer 15 may be epitaxiallygrown over the substrate 10. The epitaxial layer 15 may be doped byin-situ doping and/or ion implantation.

The intermediate semiconductor layer 20 is epitaxially grown over thesurface of the first semiconductor layer 15, and a second semiconductorlayer 30 is epitaxially grown over the intermediate semiconductor layer.Further, a mask layer 100, including a first mask layer 102 and a secondmask layer 104, is formed over the second semiconductor layer 30.

The intermediate semiconductor layer 20 is, for example, Ge orSi_((1-x))Ge_(x), where x is in a range of about 0.1 to about 0.9. Inthis embodiment, Si_((1-x))Ge_(x), is used as the intermediatesemiconductor layer 20. In the present disclosure, Si_((1-x))Ge_(x) maybe simply referred to as SiGe. The thickness of the SiGe layer 20 is ina range of about 10 nm to about 100 nm in some embodiments. In certainembodiments, the thickness of the SiGe layer 20 is in a range of about 1nm to about 20 nm, or in a range of about 2 nm to 10 nm in otherembodiments.

The epitaxial growth of the SiGe layer may be performed by using SiH₄and/or SiH₂Cl₂ and GeH₄ as source gases at a temperature in a range ofabout 500 C to 700 C and at a pressure in a range of about 10 to 100Torr (about 133 Pa to about 1333 Pa).

The second semiconductor layer 30 is, for example, Si orSi_((1-y))Ge_(y), where y<x. The second semiconductor layer is Si inthis embodiment. The Si second semiconductor layer 30 has a thickness ina range of about 20 nm to about 200 nm in some embodiments. In certainembodiments, the thickness of the Si second semiconductor layer 30 is ina range of about 50 nm to about 100 nm. The epitaxial growth of the Silayer may be performed by using SiH₄ and/or SiH₂Cl₂ as source gases at atemperature in a range of about 500 C to 700 C and at a pressure in arange of about 10 to 100 Ton (about 133 Pa to about 1333 Pa).

The mask layer 100 may include, for example, a pad oxide (e.g., siliconoxide) layer 102 as a first mask layer and a silicon nitride (SiN) masklayer 104 as a second mask layer. The thickness of the pad oxide layer102 is in a range of about 2 nm to about 15 nm and the thickness of thesilicon nitride mask layer 104 is in a range of about 10 nm to about 50nm in some embodiments.

In S102 of FIG. 1, the stacked layers of semiconductor materials withthe mask layer are patterned into fin structures 40 extending in the Xdirection, as shown in FIG. 3.

By using patterning operations, the mask layer 100 is patterned intomask patterns. The width of each of the patterns is in a range of about5 nm to about 40 nm in some embodiments, or may be in a range of about10 nm to about 30 nm in other embodiments.

As shown in FIG. 3, by using the mask patterns as etching masks, thesecond semiconductor layer 30, the intermediate semiconductor layer 20and the first semiconductor layer 15 are pattered into fin structures 40by trench etching using a dry etching method and/or a wet etchingmethod. In some embodiments, part of the substrate 10 may also beetched.

In FIG. 3, two fin structures 40 are disposed adjacent to each other.However, the number of the fin structures is not limited to two. Thenumbers may be one, three, four or five or more. In addition, one ormore dummy fin structures may be disposed adjacent to both sides of thefin structures 40 to improve pattern fidelity in patterning processes.The width (in the Y direction) of the fin structure 40 is in a range ofabout 5 nm to about 40 nm in some embodiments, and may be in a range ofabout 7 nm to about 15 nm in certain embodiments. The height in the Zdirection of the fin structure 40 is in a range of about 100 nm to about300 nm in some embodiments, and may be in a range of about 50 nm to 100nm in other embodiments. The space between the fin structures 40 is in arange of about 5 nm to about 80 nm in some embodiments, and may be in arange of about 7 nm to 15 nm in other embodiments. One skilled in theart will realize, however, that the dimensions and values recitedthroughout the descriptions are merely examples, and may be changed tosuit different scales of integrated circuits. Further, in someembodiment, at least one of the fin structures 40 is for an n-type FinFET and at least one of the fin structures 40 is for a p-type Fin FET.

It is noted that in one embodiment of the present disclosure, the widthof fin structure 40 may be set smaller than a target channel width.

In S103 of FIG. 1, the intermediate semiconductor layers 20 in the finstructures 40 are oxidized. In one embodiment of the present disclosure,the intermediate semiconductor 20 is SiGe and the first and intermediatesemiconductor layers are Si, and the SiGe layer 20 is oxidized to formSiGe oxide layers 25, as shown in FIG. 4.

Since SiGe (in particular Ge) is oxidized faster than Si, the SiGe oxidelayers 25 can be selectively formed. However, side walls of the Si firstsemiconductor layers 15 and the Si second semiconductor layers 30 mayalso be slightly oxidized to form silicon oxide layers 17 and 32.

The SiGe layer can be oxidized by an annealing or heating in anatmosphere containing oxygen (O₂), O₂ and hydrogen (H₂) or steam (H₂O).In this embodiment, wet oxidation using steam is performed at atemperature range of about 400° C. to about 800° C. for about one hourto four hours, at about atmospheric pressure. The thickness of the SiGeoxide layers 25 is in a range of about 5 nm to 25 nm in someembodiments, or about 10 nm to 20 nm in other embodiments. By thisoxidation operation, the intermediate semiconductor layers 20 (SiGe) arefully oxidized.

In some embodiments of the present disclosure, the silicon oxide layers17, 32 and part of the SiGe oxide layer 25 may optionally be removed, byusing, for example, wet etching. The etchant of the wet etching may bedilute HF. By adjusting the etching conditions (e.g., etching time), thesilicon oxide layers 17, 32 formed on the side walls of the first andthird Si layers may be removed. The SiGe oxide layer 25 is also slightlyetched.

In S104 of FIG. 1, an isolation insulating layer 50 is formed over thesubstrate 10 and the fin structures 40, as shown in FIG. 5. Theisolation insulating layer 50 includes one or more layers of insulatingmaterials such as silicon oxide, silicon oxynitride or silicon nitride,formed by LPCVD (low pressure chemical vapor deposition), plasma-CVD orflowable CVD. In the flowable CVD, flowable dielectric materials insteadof silicon oxide are deposited. Flowable dielectric materials, as theirname suggest, can “flow” during deposition to fill gaps or spaces with ahigh aspect ratio. Usually, various chemistries are added tosilicon-containing precursors to allow the deposited film to flow. Insome embodiments, nitrogen hydride bonds are added. Examples of flowabledielectric precursors, particularly flowable silicon oxide precursors,include a silicate, a siloxane, a methyl silsesquioxane (MSQ), ahydrogen silsesquioxane (HSQ), an MSQ/HSQ, a perhydrosilazane (TCPS), aperhydro-polysilazane (PSZ), a tetraethyl orthosilicate (TEOS), or asilyl-amine, such as trisilylamine (TSA). These flowable silicon oxidematerials are formed in a multiple-operation process. After the flowablefilm is deposited, it is cured and then annealed to remove un-desiredelement(s) to form silicon oxide. When the un-desired element(s) isremoved, the flowable film densifies and shrinks. In some embodiments,multiple anneal processes are conducted. The flowable film is cured andannealed more than once. The flowable film may be doped with boronand/or phosphorous. The isolation insulating layer 50 may be formed byone or more layers of SOG, SiO, SiON, SiOCN and/or fluoride-dopedsilicate glass (FSG) in some embodiments.

After forming the isolation insulating layer 50, a thermal process, forexample, an anneal process, may be performed to improve the quality ofthe isolation insulating layer 50. The thermal process may be performedbefore or after the planarization operations.

In S105 of FIG. 1, the thickness of the isolation insulating layer 50 isreduced by, for example, a planarization process including a chemicalmechanical polishing (CMP) method and/or an etch-back process, so as toexpose a part of the fin structures 40, as shown in FIG. 6. During theplanarization operations, the mask patterns 100 and a top portion of theisolation insulating layer 50 are removed. Further, by an etch-backprocess, the thickness of the isolation insulating layer 50 is reduced.

A portion of the fin structure including the exposed part 42 of the finstructure 40 becomes a channel of the Fin FET and a portion of the finstructure the embedded in the isolation insulating layer becomes a welllayer 44 of the Fin FET. The etch-back operations may be performed byusing dry etching or wet etching. By adjusting etching time, a desiredthickness of the remaining isolation insulating layer 50 can beobtained.

In FIG. 6, the SiGe oxide layer 25 is not exposed from the isolationinsulating layer 50, and the bottom of the channel layer 42 is embeddedin the isolation insulating layer 50. However, in some embodiments, theSiGe oxide layer 25 and the entire channel layer 42 may be exposed fromthe isolation insulating layer 50.

In S106 of FIG. 1, a third semiconductor layer 45 is formed over thechannel layer 42 to adjust the width of the channel of the Fin FET (inthe Y direction), as shown in FIG. 7.

As set forth above, the side wall of the first and second semiconductorlayers (Si) 15, 30 are oxidized during the oxidization of SiGe layer 20.To fully oxidize the SiGe layer 20, the thickness of the silicon oxidelayers formed over the first and second semiconductor layers (Si) 15, 30tends to become thick, which in turn reduces the width of the finstructure. In particular, the width of the upper portion of the finstructure to be a channel would be reduced.

However, by forming the third semiconductor layer 45 over the channellayer 42, the width of the channel of the Fin FET can be recovered and adesired channel width can be obtained. As shown in FIG. 7, the width ofthe channel of the Fin FET with the third semiconductor layer 45 isgenerally greater than the width of the well layer 44.

The third semiconductor layers 45 are epitaxially formed on the channellayer 42 and include silicon when the channel layer 42 is silicon. Insome embodiments, the third semiconductor layers 45 may include Si andGe, and may also include additional material such as phosphorous and/orcarbon. The third semiconductor layers 45 may be appropriately dopedwith dopants during the epitaxial growth or may not be doped.

The epitaxial growth of the Si third semiconductor layer 45 may beperformed by using SiH₄ and/or SiH₂Cl₂ as source gases at a temperaturein a range of about 500 C to 700 C and at a pressure in a range of about10 to 100 Torr (about 133 Pa to about 1333 Pa).

In the present embodiment, the third semiconductor layer 45 is made ofsilicon, and the channel of the Fin FET formed by the channel layer 42and the third semiconductor layer 45 does not contain Ge. As set forthabove, the SiGe layers 20 are fully oxidized and no SiGe semiconductorlayer remains. Accordingly, during the epitaxial growth of the thirdsemiconductor layer 45, no Ge is diffused from the SiGe layer. In someembodiments of the present disclosure, no signal corresponding to Ge isobserved at a bottom of the channel layer 42 by EDX (Energy dispersiveX-ray spectrometry), which shows that no Ge diffusion into the channellayer.

In S107 of FIG. 1, a gate structure 60 is formed over part of thechannel layers 42 of the fin structures 40. The gate structure 60extends in the Y direction and includes a gate dielectric layer 70 andan electrode layer 80. Although one gate structure 60 is formed for twofin structures in FIG. 8, two gate structures may be formed for two finstructures, respectively.

A gate dielectric material and an electrode material are formed over theisolation insulating layer 50 and the channel layer 42, and thenpatterning operations are performed so as to obtain gate structureincluding the gate electrode layer 80 and the gate dielectric layer 70.The gate electrode layer 80 is poly silicon in this embodiment. Thepatterning of the poly silicon layer is performed by using a hard maskincluding a silicon nitride layer and an oxide layer in someembodiments. The gate dielectric layer may be silicon oxide formed byCVD, PVD, ALD, e-beam evaporation, or other suitable process.

In one embodiment, a gate-last technology (a gate replacementtechnology) is employed. In the gate-last technology, the gate electrodelayer 80 and the gate dielectric layer 70 formed in the foregoingoperations are a dummy electrode layer and a dummy gate dielectriclayer, respectively, which are eventually removed.

In some embodiments, the gate dielectric layer 70 may include one ormore layers of silicon oxide, silicon nitride, silicon oxy-nitride, orhigh-k dielectric materials. High-k dielectric materials comprise metaloxides. Examples of metal oxides used for high-k dielectrics includeoxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and/or mixtures thereof. In someembodiments, a thickness of the gate dielectric layer 70 is in the rangeof about 1 nm to 5 nm. In some embodiments, the gate dielectric layer 70may include an interfacial layer made of silicon dioxide. In someembodiments, the gate electrode layer 80 may comprise a single layer ormultilayer structure.

Further, the gate electrode layer 80 may be doped poly-silicon withuniform or non-uniform doping. In some alternative embodiments, the gateelectrode layer 80 may include a metal such as Al, Cu, W, Ti, Ta, TiN,TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a workfunction compatible with the substrate material, or combinationsthereof. The electrode layer for the gate electrode layer 80 may beformed using a suitable process such as ALD, CVD, PVD, plating, orcombinations thereof. The width of the gate electrode layer 80 (in the Xdirection) is in the range of about 30 nm to about 60 nm in someembodiments.

It is understood that the Fin FET device may undergo further CMOSprocesses to form various features such as side wall insulating layers,source/drain structures, interlayer insulating layers, contacts/vias,interconnect metal layers, dielectric layers, passivation layers, etc.

The various embodiments or examples described herein offer severaladvantages over the existing art. In some embodiments of the presentdisclosure, after the channel layers are exposed from the isolationinsulating layer, an epitaxial channel layer is formed to increase thewidth of the channel of the Fin FET, thereby obtaining a desired channelwidth.

With the formation of the epitaxial channel layer to recover the channelwidth, it is possible to fully oxidize the SiGe layers into SiGe oxidewithout taking care of the loss of the channel width during the SiGelayer oxidation, which make process windows for oxidizing the SiGe layerbroader. Further, since it is possible to make the width of the finstructures (after etching) smaller, the time for fully oxidizing theSiGe layer can become shorter, thereby reducing thermal histories. Inaddition, the thickness of silicon oxide layers, which causes the lossof the channel width, can be minimized.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

In accordance with one aspect of the present disclosure, in a method formanufacturing a semiconductor device, a fin structure including a firstsemiconductor layer, an oxide layer disposed over the firstsemiconductor layer and a second semiconductor layer disposed over theoxide layer is formed. An isolation insulating layer is formed so thatthe second semiconductor layer of the fin structure protrudes from theisolation insulating layer and is exposed while the oxide layer and thefirst semiconductor layer are embedded in the isolation insulatinglayer. A third semiconductor layer is formed on the exposed secondsemiconductor layer so as to form a channel layer.

In accordance with another aspect of the present disclosure, in a methodfor manufacturing a semiconductor device, a stack of semiconductorlayers is formed. The stack includes a first semiconductor layer, anintermediate semiconductor layer formed over the first semiconductorlayer, and a second semiconductor layer formed over the intermediatesemiconductor layer. A fin structure is formed, by patterning the firstsemiconductor layer, the intermediate semiconductor layer and the secondsemiconductor layer. The intermediate semiconductor layer in the finstructure is oxidized. An isolation insulating layer is formed so thatthe second semiconductor layer of the fin structure protrudes from theisolation insulating layer and is exposed while the oxidizedintermediate layer and the first semiconductor layer are embedded in theisolation insulating layer. A third semiconductor layer is formed on theexposed second semiconductor layer so as to form a channel layer.

In accordance with another aspect of the present disclosure, asemiconductor device includes a Fin FET device. The Fin FET deviceincludes a fin structure and a gate stack. The fin structure extends ina first direction and protrudes from an isolation insulating layer. Thefin structure includes a well layer, an oxide layer disposed over thewell layer and a channel layer disposed over the oxide layer. The gatestack includes a gate electrode layer and a gate dielectric layer,covering a portion of the channel layer and extends in a seconddirection perpendicular to the first direction. A width of the welllayer in the second direction is smaller than a width of the channellayer in the second direction

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising: forming a fin structure including a firstsemiconductor layer, an oxide layer disposed over the firstsemiconductor layer and a second semiconductor layer disposed over theoxide layer; forming an isolation insulating layer so that the secondsemiconductor layer of the fin structure protrudes from the isolationinsulating layer and is exposed while the oxide layer and the firstsemiconductor layer are embedded in the isolation insulating layer; andforming a third semiconductor layer to entirely cover the exposed secondsemiconductor layer so as to form a channel of the semiconductor device.2. The method of claim 1, wherein the oxide layer includes SiGe oxide.3. The method of claim 1, wherein the second semiconductor layer includesilicon or a silicon compound.
 4. The method of claim 3, wherein thethird semiconductor layer includes silicon or a silicon compound and isepitaxially formed on the exposed second semiconductor layer.
 5. Themethod of claim 1, wherein a width of the first semiconductor layer issmaller than a width of the channel.
 6. A method for manufacturing asemiconductor device, comprising: forming a stack of semiconductorlayers including a first semiconductor layer, an intermediatesemiconductor layer formed over the first semiconductor layer, and asecond semiconductor layer formed over the intermediate semiconductorlayer, forming a fin structure, by patterning the first semiconductorlayer, the intermediate semiconductor layer and the second semiconductorlayer; oxidizing the intermediate semiconductor layer in the finstructure; forming an isolation insulating layer so that the secondsemiconductor layer of the fin structure protrudes from the isolationinsulating layer and is exposed while the oxidized intermediate layerand the first semiconductor layer are embedded in the isolationinsulating layer; and forming a third semiconductor layer to entirelycover the exposed second semiconductor layer so as to form a channel ofthe semiconductor device.
 7. The method of claim 6, wherein theintermediate semiconductor layer includes SiGe.
 8. The method of claim7, wherein in the oxidizing the intermediate semiconductor layer in thefin structure, the SiGe is fully oxidized.
 9. The method of claim 6,wherein the second semiconductor layer is made of silicon or a siliconcompound.
 10. The method of claim 9, wherein the third semiconductorlayer includes silicon or a silicon compound and is epitaxially formedon the exposed second semiconductor layer.
 11. The method of claim 9,wherein in the oxidizing the intermediate semiconductor layer in the finstructure, side walls of the first and second semiconductor layers areoxidized to form silicon oxide.
 12. The method of claim 11, furthercomprising removing the silicon oxide formed on the side walls of thefirst and second semiconductor layers.
 13. The method of claim 6,wherein a width of the first semiconductor layer in the fin structure issmaller than a width of the channel.
 14. The method of claim 6, whereinthe forming the isolation insulating layer includes: forming aninsulating material over the fin structure so that the fin structure isfully embedded in the insulating material; and removing part of theinsulating material so that the second semiconductor layer of the finstructure protrudes from the isolation insulating layer.
 15. The methodof claim 6, wherein the oxidizing the intermediate semiconductor layerincludes a thermal oxidization.
 16. The method of claim 6, furthercomprising forming a gate structure over the channel.
 17. A method formanufacturing a semiconductor device, comprising: forming a stack ofsemiconductor layers including a first semiconductor layer, anintermediate semiconductor layer formed over the first semiconductorlayer, and a second semiconductor layer formed over the intermediatesemiconductor layer, forming fin structures, by patterning the firstsemiconductor layer, the intermediate semiconductor layer and the secondsemiconductor layer, such that each of the fin structures including thefirst semiconductor layer, the intermediate semiconductor layer and thesecond semiconductor layer; oxidizing the intermediate semiconductorlayer in each of the fin structures; forming an isolation insulatinglayer so that the second semiconductor layer of each of the finstructures protrudes from the isolation insulating layer and is exposedwhile the oxidized intermediate layer and the first semiconductor layerof each of the fin structures are embedded in the isolation insulatinglayer; and forming a third semiconductor layer to entirely cover theexposed second semiconductor layer of each of the fin structures. 18.The method of claim 17, wherein in the oxidizing the intermediatesemiconductor layer in the fin structure, side walls of the first andsecond semiconductor layers are oxidized to form silicon oxide layers.19. The method of claim 18, wherein the silicon oxide layers areremoved.
 20. The method of claim 19, wherein when removing the siliconoxide layers, a part of the oxidized intermediate layer is removed.